Rotational molding or more commonly termed rotomolding, is a process whereby plastic parts can be manufactured. These parts are created by starting with a thermoplastic resin, usually introduced into the interior of a mold in a powder form. The mold is then heated using a convection heating or radiation process. The mold is rotated on two axes, typically perpendicular to one another. As the plastic heats it is also tumbled inside the mold. The plastic eventually reaches a temperature at which it fuses to the sides of the mold and to itself and forms a shell. The exterior of the shell of plastic retains the shape of the interior surface of the mold. The shell has a wall thickness that is somewhat uniform, although this uniformity is affected by the shape of the mold and the processing techniques used. The thickness of the part is determined by the amount of resin introduced into the mold.
In a typical rotomolding system, a mold is mounted at the end of a holding device, generally at the end of an arm. The arm is mounted at the opposite end to a hub. This hub may have several similar arms located evenly spaced around the hub, on the same plane, described by the axis of the arm. At the opposite end of the arm, the mold is mounted in any of a variety of ways. Sometimes the mold is mounted in a framework of tubing, also known as a spider, which allows multiple molds to be mounted and processed simultaneously, or it is mounted by itself. The holding device on which the mold is mounted has the capability to be rotated on two axes, perpendicular to one anther. Also the hub is able to rotate the entire arm assembly through various stages of the molding process. Generally, in a production environment, these states are: loading, heating (oven), cooling, and unloading.
This describes a common turret-style machine used in production of rotomolded parts. There are a variety of other styles of rotational molding machines that allow the mold to produce plastic parts. Other machines may have different mounting methods, perhaps a different description of rotation is imparted by the machine due to the unusual shape of the parts, as in a “rock and roll” machine. While other machines may not have the multiple arms described here, as in a single station or laboratory styles, rotomolding machine, the invention described herein, is applicable to any of these machine styles, but for purposes of a clear description of this invention, the turret-style machine will be used here as the exemplary apparatus.
While the equipment for rotomolding does have a variety of styles, the molding process, to which this device is applicable, is essentially the same. This device and its method of use can be used in any of these style machines.
The rotational molding process can be divided into four stages or steps: 1) loading, 2) molding involving heating, fusing and rotation, 3) cooling, 4) unloading; each step generally defined as follows:    1) Loading: Prior to the molding process, the mold is opened and charged with a predetermined weight of raw material, plastic resin that is ground into a powder consistency. The mold is then shut and bolted or clamped closed;    2) Molding: The mold is positioned in the oven. As the oven is brought up to a programmed temperature, the mold is rotated slowly on two axes of rotation to allow the plastic to tumble around the interior of the mold cavity; the tumbling is the effect of gravity as the raw material is raised inside the mold from the rotation. The rise in temperature inside the mold cavity causes the resin to fuse evenly to the sidewalls of the heated mold;    3) Cooling: After a predetermined amount of time rotating in the oven, the mold is pivoted or moved into a cooling station. The mold is cooled by air or water spray to a temperature that allows the mold to be opened;    4) Unloading: The mold is split apart and the part is removed from the mold.
To form a molded article, a moldable material, typically a powdered resin of a predetermined weight is loaded into the mold. During the molding process, the mold is placed in a heated environment of sufficient temperature to fuse the powdered resin onto the internal walls or interior surfaces of the mold. As the mold rotates, the powder drops onto the opposite wall by gravitational effect. The rotation of the mold is not fast enough to impart centrifugal force onto the plastic resin; therefore, the tumbling effect on the resin can coat the mold to a sufficiently accurate wall thickness to create a uniform plastic product.
Rotational molds are fabricated having internal uniform walls or surfaces forming hollow, thin-walled molds representing the finished shape of the product manufactured. The mold can be aluminum, sheet metal, or other materials which allow the conduction of heat from the oven into the interior cavity of the mold.
During the rotomolding process, the interior of the mold will build up pressure. As the mold is coated with plastic, by heating and tumbling through the bi-axle rotation, the pressure building up inside the interior cavity has to be vented to the atmosphere. If not, the part may develop “blow holes” where the pressure escapes through weak areas of the part, usually around the parting line of the mold. Also, a vacuum can occur during the cooling cycle which will result in a part that collapses or deforms as the air pressure drops below the atmospheric pressure.
To provide venting to such molds, most rotomolders use a hollow tube that is fixed to the outside of the mold, projecting into the mold perpendicular to the surface to which the tube is mounted. The tube is usually stuffed with a packing filter media like fiberglass, wool or steel wool, which allows the gas to escape but keeps the powdered plastic inside the mold. This practice has its drawbacks. Often different operators will pack the tubes with filter media too tightly; this results in excessively high pressure in the cavity and results in “blow holes.” If the media is packed too loosely, it can come out of the mold. If not changed in a timely manner, the plastic resin will occlude the packing filter media in the tube and resulting pressure rise will lead to the “blow hole” problems listed above.
To address this problem, Alden C. Boyce invented a special vent tube system which is described in U.S. Pat. No. 6,280,176 granted Aug. 28, 2001. The Boyce vent tube employed an elongated central bore in a probe-like rod divided into three sections. Counter bores in conjunction with the central bore provides an air escape passageway, which communicates with the interior of the mold via plurality of pore-like holes arranged in a band just above the innermost top of the vent. To solve the packing material problem of prior art vent tubes, Boyce provided small beads in the central bore which were retained by a perforated disk and retaining clip. This Boyce tube vent purportedly eliminated the need to service the vent by cleaning and repacking the vent after every mold cycle.
The present invention provides a unique venting device that not only eliminated the clogging and vent servicing requirements, but also enables a unique and superior method of rotational molding to be achieved.